This project investigates the role of genetic factors in primate vocal development. Research strategies employed in these studies include analysis of vocal traits in hybrid offspring having vocally distinctive parents, comparing the vocal traits of full and half sibs with unrelated age-matched infants, and analysis of the effects of differential rearing on normal vocal development. In FY 86, a long-term project studying the inheritance patterns of the species-typical isolation call (Isolation Peep; IP) in hybrids of crosses involving vocally distinctive sympatric populations (Peruvian Gothic x Peruvian Roman) was completed. Hybrids studied until 2 years of age produce IPs with acoustic parameters that are intermediate to IPs from the parents, but whose overall structure more closely resembles that of the Roman parent. A comparative study of the isolation calls of prosimian primates (family Lemuridae) was also completed, following the successful recording of the isolation calls from a second Lemur mongoz infant, along with representative samples from both parents. The data from this study show that acoustic parameters differentiating the isolation calls of different lemur species are present in the newborn infant and persist into adulthood. Adult members of the genus Lemur engage in vocal dialogues with their separated offspring, using adult versions of the infant's isolation call. A study begun this year is focused on examining the vocal characteristics of primate twins, with the goal of developing a primate model for the study of shared vocal traits in twins. Five species of prosimians and two species of marmosets-- all having a high incidence of multiple births-- were studied, and the species-specific attributes of their isolation calls determined. The infants of 2 nocturnal prosimians (mouse lemur, Microcebus murinus; dwarf lemur, Cheirogaleus medius) produce isolation calls in the 18-20 kHz range and are more vocal during their active cycle in a test environment illuminated with long wave length light.